I will only deal with the most famous top of the line models. To do otherwise would require a pamphlet.The question has to do with whether or not you want to keep your Laserdiscs or replace them (if they can be replaced) with DVDs.
For image quality, there are two ways to divide the best Elite players; those that yield the best, most film like composite signal and those that used advanced comb filters and digital noise supression to yield an excellent S-Video signal. The best composite signal is delivered by the LD-S2 followed very closely by the CLD-97. However, the only place the filmlike quality of either is really obvious is on a CRT display. If you have a high resolution digital display the quality of the comb filter and signal upsampling on that display become the most important factors. With a good scaler either the S2 or 97 can deliver quality images using the composite output. Virtually any scaler is going to have a better comb filter than these devices so using the S-Video filter should be unwise. Nevertheless; try it. Let your eyes tell you which is better. If you have no scaler and upsampling on your display is not that good, the S-Video out of a 97 would yield a better image. The 97 has automatic side change; the LD-S2 does not. It was designed as a pure Laserdisc machine, and was the father to the chassis design of the X0 and X9 Muse machines. In other words, its RF and EMI rejection is excellent and it is a massive device (almost 70 pounds). For the 97 to match the film like quality of the LD-S2 on CRT displays the digital noise suppression must be turned off.
The CLD-99 and the Japanese S9 are quite similar to each other and use even more advanced comb filters, better than some of the scalers currently on the market. These are also the only machines that come from the factory with AC-3 (Dolby Digital) out and the 99 was the first top of the line Elite to also play CDs.
All of the players do DTS via the optical cable out. Whether or not the later players are better than an upscaled LD-S2 (on a good scaler) in terms of image is a difficult question and very dependent on other elements in the chain.
Just remember, even the best Laserdisc players are still playing a composite analogue Disc. That means that they are not close to the resolution of Blu-ray or HD DVD no matter what the display methodology is. In fact, Laserdisc resolution is about the same as S-VHS but with a much more stable image and better colour.
If you have Laserdiscs that cannot be replaced these are good machines if your current machine dies. There are others that are as good or better (Runco comes to mind), but they are extremely rare and very expensive.
No matter what you do, the picture will not look any sharper than a progressive scan DVD on the same equipment though they all have a discinct lack of those DVD artifacts that really bother some people. Again, digital noise suppression should be turned off.
Also, laserdisc players really hate to sit unused. Even if you aren't going to watch it, it is a good idea to run one movie a month to keep the motor and other mechanical bits lubricated. The replacement motor on an LD-S2 (or an X0 or X9) is frightfully expensive, in part because it is so substantial. And do not count on your local dealer knowing anything about the machine. There are people who do this work who are superb and they can be found on the internet.
DTS soundtracks on Laser are not inherently better than DVD but are in practice because standard DVDs cut the sound bandwidth in half to save room for special features. Blu-Ray and HD DVD have suprelative sound and the new lossless standards and should be in a different class. However, stats are stats and sound is sound. Goldeneye or Independence Day on Laser has a terrific soundtrack by any standard.
Finally, If you decide on an LD-S2 or CLD-97, make sure you try to find one that already has AC-3 (DD) added. If you have to do it the cost may double. However, if you can find one that is already modified the cost of the player and a used scaler can be cheaper than a CLD-99 (still running $500 or so) and may yield better performance than the 99 alone.